The present invention relates to an improved apparatus for removing solvent from bulk or finite heat-sensitive products and pharmaceuticals.
In numerous chemical processes, particularly in the pharmaceutical industry, the drying stage is often the last of a long series of individual operations; this stage is thus important both because it is carried out on a product which already has a considerable added value and because it has a notable influence on the final characteristics and appearance of the product.
The solvent-wet products normally come from a previous stage of centrifugation or filtering (filter press). It is the current practice to vacuum dry the product at the highest temperature which is permitted without the risk of thermal degradation of the product. The need of a high temperature comes from the need to reduce the drying time to acceptable values. This kind of approach to the problem of drying of the above products leads to the result that an intense and strong evaporation occurs during drying, which often causes a carry-over of the solid product's particles by the vapour flow. This carry-over could cause a clogging of the suction filters and problems as far as the vacuum pump is concerned. Moreover, when a high drying temperature is used, the product could become pasty and could need more time to be dried. Another problem, particularly when a rotary vacuum drier is used, is that the product forms particles agglomerates, referred to as "lumps" below, which, in most cases, make the dried product not suitable for the final utilization.
Therefore, the product coming from the drier needs a further granulation and milling which increases the final cost of the product itself.
After drying, the product should have as low a residual liquid content as is compatible with the optimum parameters of the operation, and should also have as uniform a grain size as possible, not only for reasons of homogeneity but particularly because a non-uniform grain size denotes the presence of the above mentioned lumps, which retain a large percentage content of liquid within them during drying and after discharge from the drier, rendering the process partially ineffective. In the drying of micro-crystalline, amorphous, fibrous or gel type organic solids in conventional rotary driers, it is noted that it is difficult to eliminate lumps from the final product. Moreover, in the case of organic solids in which the drying process is controlled by diffusion of the liquid through the solid, the duration of the process will be longer the larger the lumps.
Another problem relates to the surface layers of the particles which, whenever the drying process is controlled by the diffusion of the liquid through the solid, tend to dry more rapidly than the inner ones whereby a relatively impenetrable, dry crust is formed which prevents further drying of the interior of the particles themselves.
Accordingly, it is an object of the invention to provide a drying method for heat-sensitive products and pharmaceuticals which solves the above problems and which permits to avoid the stages of milling and/or granulation and which gives a final dried product in a powdered form.